According to a U.S. Department of Health and Human Services investigation, many doctors listed as serving low-income patients either can't offer appointments at all or have months-long wait times.

About half of Medicaid doctors in managed-care networks can’t offer appointments to patients because they don’t have availability or their status with the health plan isn’t accurate, according to a report released Tuesday by the Department of Health and Human Services (HHS) Office of Inspector General.

In cold calls with doctors listed in the directories of privately managed Medicaid plans, 35 percent weren’t practicing at their listed location, 8 percent were no longer participating in the plan and 8 percent weren’t seeing new patients, according to the report. The report also found that 28 percent of doctors have wait times of more than a month, and 10 percent had wait times longer than two months.

Most states require participating providers to offer appointments within a certain timeframe, and for most it’s a month or less, according to the report.

HHS Inspector General Daniel Levinson argued in the report that the findings raise doubts that managed care plans are maintaining sufficiently robust doctor networks and meeting other areas of state compliance.

“When providers listed as participating in a plan cannot offer appointments, it may create a significant obstacle for an enrollee seeking care,” he wrote. “Moreover, it raises questions about the adequacy of provider networks -- the actual size of provider networks may be considerably smaller than what is presented by Medicaid managed care plans.”

Medicaid managed care organizations are privately run institutions that states pay to take over much of their administrative work, including maintaining networks of doctors.States are increasingly turning to them in hopes they can better coordinate care and hold down costs. An estimated 76 percent of all Medicaid patients will be enrolled in managed care plans by the end of 2016, according to the consulting firm Avalere Health.

All states are required to have a written strategy for assessing managed care plans that includes standards for access to care. The newly released Inspector General report comes three months after another HHS report on access through Medicaid managed care that found wide variation in the strictness of state standards, little in the way of tracking violations and sanctioning plans that are underperforming. One example: Minimum provider density varies from one in 100 to one in 2,500 Medicaid patients.

The report has stoked an initiative within the federal government to step in and force tougher standards. HHS is expected to release those standards in January. But groups such as the National Association of Medicaid Directors and Medicaid Health Plans of America (MHPA), a trade association, have argued states need flexibility because of regional variations in the number of doctors available and other differences.

The MHPA pushed back against the latest Inspector General report. Jeff Myers, the group’s president, said there’s no question provider directories should be up to date, but most patients don’t get their information from the directories they receive when they sign up for a plan. The Centers for Medicare and Medicaid Services, which administers Medicaid, supported that view in the report, saying enrollees also have call centers, care coordinators, ombudsman offices and community health centers as resources for information.

He also pointed out that recent surveys and studies show improving and wide satisfaction with doctor access among Medicaid patients. The Inspector General study should compare those experiences with those of patients using traditional Medicaid or a plan from the federal exchange, which has taken criticism for narrow networks, he argued.

“It focuses on the directories for managed care but it doesn’t put it in context,” Myers said. “It doesn’t look at the wait times that a hypothetical patient would have in a fragmented, fee-for-service system, which I expect would be worse, nor does it look at the [federal exchange].”

The HHS report was conducted in the latter half of 2013, before Medicaid expansion that has added some 9 million enrollees to the system. It remains to be seen whether new volume will translate into access problems.